I agree with scottychaos, that tank is pretty small and I would avoid including anymore fish. If algae is your problem, I'd consider doing more water changes (you should be changing at least half the water once a week for this small tank IMO), and scrub the walls and so forth to remove algae. Algae is many times a sign that there are too many nutrients in the tank, water changes are important to reduce their levels...is your tank cycled? Ammonia especially is great for growing algae.
Also, reduce the amount of light exposure your tanks gets: reduce the overhead light on the tank (get a timer, 8 hours is more than enough), and get it away from any sunlight.
IME, algae eating fish like otocinclus also have a tendancy to produce a lot of waste, and produce more waste than other fish their size. They can dirty up a tank pretty well. If I had to choose an algae eater for your tank, I would go with some tiny snails, like ramhorns or malaysian trumpet snails (MTS).
I hope this helps.
The outcompeting thing is a myth. It's been debunked, but it keeps spreading around the hobby. I used to buy into it too, but it never really made any sense to me. Turns out it didn't make sense for a reason. Just making people aware... best to prevent new hobbyists from spreading it after reading this thread.
Once algae has bloomed, it does feed on the tank's nutrients of course... it's what encourages those spores to bloom in the first place that's the issue. There are a ton of high-light high-nutrient tanks out there with little to no algae, though you can bet the spores are in there. With the out-competing concept out the window, that leaves a trigger as the culprit. I can't say for sure whether ammonia (actually ammonium) is the only trigger, but it is the primary one.
CO2 does make the tank less hospitable to some types of algae. But not all.
thanks for the info all. I actually do have a filter box on this tank. It is nothing fancy, a $20 nano unit. For light I had a floro light on there (compact running at 10w) and the algae went insane and the plants didn't do well at all. I switched to a small halogen desk lamp and the plants are growing again and the algae isn't as bad.